I added the following feature to the OpenML API: Whenever there is an error in the dataset (this can be anything: features are not legal, meta-features can not be calculated, arff can not be parsed) this value is shown in the xml.

New XSD schema definition:
https://www.openml.org/api/v1/xsd/openml.data.upload

This does not change the current functionality of the API, but it would probably be good to raise a warning when the error / warning field is not empty.